Flow line electric impedance generation

ABSTRACT

A flowline electrical impedance generation apparatus for generating a local electrical impedance in a metallic tubing portion of a flowline may be used in place of insulation joints in various systems. The impedance generation apparatus is tuned or tuneable to achieve maximum impedance at chosen frequencies, i.e., frequencies which it is desired to block.

This invention relates to flow line electrical impedance generation, in particular to devices for generating electrical impedance in flow lines and communication apparatus and systems for communicating in flow line structures where tubing portions of the flow line structure are used as part of a signal path.

The present methods and apparatus are of particular interest in the oil and gas industry where flow lines are used for transporting product (oil and/or gas) up out of wells and away from wells either along, for example, a sea bed or along the land surface. In each situation metallic tubing is provided through which the product flows, be this, for example, “casing”, “lining”, or “production tubing” down hole in a well or a “pipeline” along a seabed or the earth's surface. In this specification the word “tubing” is used to cover all such metallic tubing.

In various situations it can be desirable to provide an insulation joint between two lengths of tubing so that a first of the lengths of tubing is electrically isolated from a second of the lengths of tubing. As a particular example, there are various existing systems for communicating in flow line systems where the metallic structure of the flow line, that is to say the tubing, is used as a signal channel for carrying electrical signals. Such communication systems are useful in allowing the transmission of data from, for example, down hole in a well to the surface. Such data may relate to parameters which are measured in the well such as pressure and/or temperature.

Insulation joints can be used in such communication systems to provide a mechanism for applying signals onto the metallic structure within the well or extracting signals from within the metallic structure within the well. In particular some form of transmitter or receiver may be connected across the insulation joint.

An example of such a communication system using this type of mechanism to extract signals from and inject signals into a subsea oil pipeline installation is described in one of the inventor's earlier patents U.S. Pat. No. 5,587,707.

Whilst transmitting or receiving signals across an insulation joint can work well in many circumstances there are situations where it is not possible to introduce an insulation joint into such tubing. It will be appreciated that in at least some circumstances there can be significant pressures or other loads which will be experienced by the tubing in flow lines used in the oil and gas industry and therefore the introduction of an insulation joint can be undesirable or impossible. The introduction of such a joint may degrade the structural integrity of the metallic tubing.

It is an aim of the present invention to provide a way to avoid the introduction of a physical insulation joint whilst still giving at least some of the functionality that may be given by a physical insulation joint.

According to the present invention there is provided flowline electrical impedance generation means for generating a local electrical impedance in a metallic tubing portion of a flowline.

There may be a flow line arrangement comprising a metallic tubing portion and impedance generation means as defined above.

The impedance generation means may be arranged to electromagnetically generate the local electrical impedance. The impedance generation means may be arranged so as to not impair the structural integrity of the tubing with which it is used. The impedance generation means may be structurally distinct from the tubing with which it is used. The impedance generation means may be arranged to generate the local electrical impedance in the tubing without modifying the dimensions or materials of the tubing in that region.

The impedance generation means may be arranged to be mounted on the tubing portion. The impedance generation means may be arranged to be mounted around the tubing portion. The impedance generation means may be arranged so that tubing may run uninterruptedly through or past the impedance generation means when the apparatus is installed and in use.

The impedance generation means may be arranged so that the value of the local electrical impedance is dependent on frequency. The impedance generation means may be arranged so that the value of the local electrical impedance is dependent on frequency and will exhibit a maximum within a predetermined range of frequencies. This means that the impedance generation means may be constructed so as to generate a relatively high impedance to signals within a chosen frequency range and a lower impedance to signals outside this range.

Preferably the impedance generation means is arranged to generate an impedance which is tuned or tuneable to a chosen frequency of signals to be seen by the local impedance.

The impedance generation means may comprise a generally toroidal portion of magnetic material for surrounding the tubing portion. A winding may be provided on the toroidal portion of magnetic material. This can allow the tubing portion to act as a single turn winding in a transformer also comprising the toroidal portion of magnetic material and said winding provided on the toroidal portion of magnetic material.

Here it is to be understood that word toroidal is used in a broad way to refer to any ring like shape that can encircle a length of tubing—it is not relevant what shape the ring adopts nor is it relevant what shape a cross-section through the material of the ring has.

The winding may be connected to at least one impedance component. The at least one impedance component may be chosen so that the impedance seen in a tubing portion passing through the toroidal portion of magnetic material varies with frequency. The impedance component may comprise a capacitor connected in series with the winding.

According to another aspect of the present invention there is provided flowline communication apparatus for use where metallic tubing of a flowline system is used in a signal path, the apparatus comprising impedance generation means as defined above for generating a local electrical impedance in a metallic tubing portion and a communications unit comprising at least one of a transmitter for transmitting signals into the tubing portion across the local electrical impedance and a receiver for receiving signals across the local impedance from the tubing.

According to another aspect of the present invention there is provided a flowline communication arrangement comprising flowline communication apparatus as defined above and a length of tubing.

Preferably the impedance generation means is arranged to generate an impedance which is tuned or tuneable to the frequency of the signals to be transmitted and/or received across the local impedance. Here the idea of tuning the impedance relates to providing a maximum impedance at the signalling frequency to help in the application and/or extraction of signals across the impedance.

The flowline communication apparatus may comprise a control unit for controlling the impedance generation means to control the local electrical impedance. Where there is no communication apparatus, the impedance generation means may have an associated control unit. The impedance generation means may comprise a control unit. The control unit may be arranged to tune the impedance to the frequency of signals being sent and/or received. The control unit may be arranged to selectively enable and disable the impedance generation means to control whether a local electrical impedance is generated.

Where the impedance generation means comprises a toroidal portion of magnetic material carrying a winding, the impedance generation means may further comprise a plurality of impedance components which are selectively electrically connectable, for example under control of the control unit, with the winding to alter a resonant frequency of the system and hence tune the local electrical impedance. There may be a plurality of capacitors selectively connectable in series with the winding. The capacitors may be connected in parallel relative to each other. The capacitors may be connected such that each, or each of a subset of the capacitors, can be selectively and independently switched into series connection with the winding. The capacitors may be arranged in a ladder network.

The impedance generation means may comprise active and/or passive components.

The control unit may be arranged to measure one of: received signal strength from the tubing and the local electrical impedance in the tubing, and arranged to control the impedance generation means so that at the signalling frequency, the signal strength or impedance respectively tends towards a maximum.

The flowline communication apparatus may have a spaced pair of electrical contacts for contacting with tubing so as to connect the transmitter and/or receiver across the local impedance. The apparatus may have one electrical contact disposed on a first side of the toroidal portion of magnetic material and another electrical contact disposed on a second side of the toroidal portion of magnetic material.

The tubing may be used in the signal path as a transmission medium and/or as an antenna.

Where the impedance generating means is mounted on or around tubing, insulation may be provided on the outer surface of the tubing in regions on both sides of the impedance generating means. Where tubing on which the impedance generating means is mounted is itself provided within a second length of tubing, insulation may be provided between the two lengths of tubing in regions on both sides of the impedance generating means.

Typically the tubing will be downhole tubing or pipeline tubing as used in the oil and gas industry. Typically the tubing will be for carrying fluid, generally oil and/or gas.

The flowline communication apparatus may be used in communicating between metallic tubing in a main bore hole and metallic tubing in a lateral which is not electrically connected to the metallic tubing in the main bore. The communications apparatus may pick up signals transmitted through the formation in which the communications apparatus is disposed.

The flowline communication apparatus may be used as a relay station for both receiving signals from and transmitting signals into tubing.

The flowline communication apparatus may be used in drill stem testing.

The impedance generation means may be used to block or impede signals in a riser. The impedance generation means may be provided in a well installation including a communication system arranged to transmit signals at a predetermined frequency and having a riser leading away from a well head, the impedance generation means being disposed and arranged so as offer impedance to transmission of signals of said predetermined frequency from the well head into the riser. The impedance generation means may be disposed around the riser. The impedance generation means may be tuned to said predetermined frequency.

According to another aspect of the present invention there is provided a flowline power transmission apparatus set comprising a master unit arranged for applying power to metallic tubing of a flowline system and at least one other unit arranged to extract power from metallic tubing of a flowline system, the at least one other unit comprising an impedance generation means as defined above to generate an impedance across which power can be extracted in use.

The master unit may be a master communications unit.

The at least one other unit may be a communications unit. The communications unit may have one or more of the respective optional features defined above.

The master unit may be arranged to apply power signals having a predefined frequency. The master unit may be arranged to selectively apply power signals having one of a plurality of predefined frequencies.

The at least one other unit may be arranged to extract power only when a signal having a predefined frequency is detected at said other unit.

There may be a plurality of said other units each of which has an assigned predefined frequency and each being arranged to extract power only when a signal having the respective predefined frequency is detected at that unit.

The or each other unit may be arranged to activate the impedance generation means to allow the extraction of power under predetermined conditions. The predetermined conditions may be the detection of a signal having a respective predefined frequency.

The impedance generation means in the or each other unit may be tuned or tuneable to said respective predefined frequency, so as to provide high impedance to signals having that frequency.

The master unit may comprise power generation means. The power generation means may comprise a turbine. The master unit may comprise impedance generation means for generating an impedance across which power can be applied in use.

According to another aspect of the invention there is provided a flowline power transmission system comprising a flowline power transmission apparatus set as defined above and a flowline on which the apparatus set is installed.

The flowline may comprise a horizontal completion in an oil and/or gas well.

According to another aspect of the present invention there is provided flowline communication apparatus for use where metallic tubing of a flowline system is used in a signal path, the apparatus comprising a toroidal portion of magnetic material for location around a length of metallic tubing and a winding wound around the toroidal portion of magnetic material and connected to at least one impedance component chosen so that an impedance seen in a metallic tubing portion passing through the toroidal portion of magnetic material varies with frequency and a communications unit comprising at least one of a transmitter for transmitting signals into the length of metallic tubing across a portion of the tubing passing through the toroidal portion of magnetic material and a receiver for receiving signals, from the length of metallic tubing, across a portion of the tubing passing through the toroidal portion of magnetic material.

According to another aspect of the present invention there is provided a flowline communication system comprising a length of tubing and flowline communication apparatus as defined above with the communications unit disposed for transmitting signals into the tubing and/or receiving signals from the tubing at a first location and comprising another communications means for transmitting signals into the tubing and/or receiving signals from the tubing at a second location.

According to another aspect of the present invention there is provided a well installation comprising metallic structure including a length of tubing and flowline impedance generation means as defined above.

According to another aspect of the present invention there is provided a downhole communication system for communicating between metallic tubing in a main bore hole and metallic tubing in a lateral bore hole which is not electrically connected to the metallic tubing in the main bore, comprising transmitting means for applying signals to the metallic tubing in the main bore so that signals pass into the surrounding formation and towards the lateral bore and flowline communication apparatus as defined above provided in the lateral bore for extracting signals from the tubing in the lateral bore, the signals in the tubing in the lateral bore having been generated by the signals passing through the surrounding formation.

According to another aspect of the present invention there is provided a drill stem testing system comprising, a length of metallic tubing supporting a drill bit, a downhole sensor for sensing a downhole parameter, and a communication system for transmitting data from the downhole sensor to the surface, the communication system comprising flowline communication apparatus as defined above for injecting signals representing said data into the drill supporting metallic tubing for transmission towards the surface.

Embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

FIG. 1 schematically shows a well installation including a lateral bore and a communication system for communicating between the main bore and the lateral bore;

FIG. 2 schematically shows part of the communication system of the well installation shown in FIG. 1; and

FIG. 3 shows part of the arrangement shown in FIG. 2 in a schematic circuit diagram form;

FIG. 4 shows one particular implementation of the circuit components shown in FIG. 3;

FIG. 5 shows another particular implementation of the circuit components shown in FIG. 3;

FIG. 6 shows a drill testing system comprising a communication system;

FIG. 7 schematically shows a subsea well installation with a communication system;

FIG. 8 schematically shows another well with a communication system; and

FIG. 9 shows a communications unit of the system shown in FIG. 8.

FIG. 1 schematically shows a well installation which well has a main bore 1 and a lateral bore 2. As is well known in the oil and gas industry when a well is drilled, holes are drilled into the formation and these are lined with metallic tubing in one form or another to form a flow line through which product from the well may pass up the well to the surface.

The metallic tubing provided within the well may take various forms. It may for example, be an outer casing and within it an inner production tubing or drill stem tubing. In other circumstances there may simply be a liner tubing which is provided in the bore hole with no further tubing within it. All of these different possible configurations might be used with the ideas of the present invention and the details of such configurations is not of particular interest in the present application. The present invention is of interest in any of these situations where there is metallic tubing be that “casing”, “lining”, “production tubing”, “drill stem tubing” or so on. Thus, whilst the word “tubing” can sometimes have a special meaning within the oil and gas industry, within this specification it is used generically to refer to any tubular like length of metallic material.

In the well installation shown in FIG. 1, tubing in the form of a casing 11 is provided in the main bore hole 1 and tubing in the form of a liner 21 is provided in the lateral bore 2. Where the lateral bore 2 joins into the main bore hole 1, there is an opening or breakout in the casing 11 of the main bore 1. This opening is, of course, there to allow product from the lateral liner 21 to pass into the casing 11 for upwards transport to the surface. In the present case and in general, there is no metal to metal contact between the casing 11 in the main bore hole and the liner 21 in the lateral. Rather the flow path for the product is completed by cementing in the end of the liner 21 in the region where it meets the casing 11. Such portions of cement 3 are schematically shown in FIG. 1.

Thus there is a continuous conduit for product to flow from the lateral bore 2 into the main bore 1, i.e. within the liner 21 and casing 11, but there is no metal to metal contact between the liner 21 and the casing 11. Thus, there is no ready path for electrical signals between the liner 21 and casing 11. This can present difficulties when using communication systems which rely on the transmission of electrical signals through the metallic structure of the well. This is because there may be a desire to transmit signals between equipment located in the lateral bore 2 and equipment located in the main bore 1 and/or the surface.

A potential solution to this problem is to look to detect signals passing out into the formation F surrounding the bore holes due to signals being transmitted into the metallic structure 11, 21. In this example, we considered a case where signals are injected into the casing 11 at a region near where the lateral bore 2 joins the main bore 1 and from there (amongst other things) propagate out into the formation F.

In the installation shown in FIG. 1, a downhole communication tool 4 is provided in the casing 11 at a location near to where the lateral bore 2 joins the main bore 1. This downhole communication tool 4 is arranged for applying high current, very low frequency, signals into the casing 11 via spaced contacts 41. Such downhole tools are commercially available from the Applicant. In a normal mode of operation, such a tool is used to inject high current, very low frequency, signals into the metallic structure 11 from where they propagate along the metallic structure to another location where they may be detected by a similar tool or at the surface. However, as this process occurs, electro-magnetic signals E (which are only highly schematically represented in the drawings) will also travel away from the tool 4 into the formation F surrounding the tool 4. (Of course, other techniques may be used to inject signals into the formation F in the region of the meeting point between the main bore 1 and the natural bore 2.) This brings another possibility of using the portion of the liner 21 close to the main bore 1 as an antenna for picking up these signals.

A natural way to do this would be to introduce an insulation joint in the length of tubing 21 near the main bore 1 so that there are two portions of metallic tubing which are electrically insulated/isolated from one another. In such a circumstance a receiver (or a transmitter) may be connected across the insulation joint to allow the reception (or transmission) of signals. However, this is a circumstance where including an insulation joint in the tubing is highly undesirable or impossible. This is due to the high loads which will be exerted on the tubing 21 as it is pushed into the lateral bore 2 to form the liner 21.

In the present communication system as shown in FIG. 1 rather than including an insulation joint in the liner 21, flow line impedance generation means 5 is provided for generating a local electrical impedance in the liner 21 which is much higher than the impedance of the tubing alone. This gives rise to the possibility of receiving signals across this local electrical impedance created in the liner 21 (and also transmitting signals across that impedance).

In the present communications system, a communications unit 6 is provided which comprises the flow line impedance generation means 5 and a transceiver 61 for transmitting and receiving signals across the impedance which can be generated by the impedance generating means 5. The communication unit 6 also comprises spaced contacts 62 for contacting with the metallic tubing 21 on either side of the impedance generating means 5 and thus on either side of the local impedance generated by the impedance generating means 5 in use.

FIG. 2 schematically shows the communication unit 6 in more detail. In the present embodiment the transceiver 61 and impedance generation means 5 are controlled by a control unit 63. This control unit 63 is used in the present embodiment to control the transmission and reception of signals from and to the communication unit and might for example, also receive data from local sensors to be included in messages to be transmitted.

Further, in the present embodiment the control unit 63 controls the behaviour of the impedance generation means 5. In particular, it is arranged to control the frequency at which the impedance generated by the impedance generation means is maximum. This is done so that the local electrical impedance generated by the impedance generation means 5 is most effective at the frequency of the signals which are to be transmitted and/or received.

During reception of signals, maximisation of the signals received may be achieved by monitoring the received signal strength, varying the characteristics of the impedance generation means 5 across a range and choosing the characteristics of the impedance generation means 5 which give rise to the largest received signal strength. As an alternative, the characteristics of the impedance generation means 5 may be directly controlled in response to a known or determined frequency of the signals to be transmitted/received. As a yet further alternative, the impedance actually generated in the portion of tubing passing through the impedance generation means 5 might be measured and the characteristics of the impedance generation means 5 varied until this value is maximised.

It should also be said that in other alternatives the impedance generation means 5 may not have this element of control and rather just be designed and arranged to give a maximum impedance at a pre-chosen frequency. To put this another way, it is possible to either have the impedance generation means 5 pre-tuned to give its best effect at a pre-chosen frequency or it is possible to have the impedance generation means 5 “tuneable” so that it may be actively “tuned” in use.

In the present embodiment, the impedance generation means 5 comprises a toroidal portion of magnetic material 51 which is located around the tubing 21 in which the impedance is to be generated. In practice this portion of magnetic material 51 may have any shape which is suitable for surrounding the tubing 21. It is shown only in highly schematic form in FIG. 2. There is no requirement for the portion of magnetic material 51 to have an overall circular shape or to have any particular cross section. Moreover, the toroidal portion of magnetic material 51 may be originally two half annular pieces of material which are joined together around the tubing 21 or any other number of pieces which are joined together around the tubing 21.

Furthermore, there may in fact be a multiple number of toroidal pieces of magnetic material which are arranged axially next to one another along the tubing 21.

In this embodiment a multi-turn winding 52 is wound around the portion of magnetic material 51 and connected in series with at least one impedance component 53. With this arrangement the winding 52, magnetic material 51 and tubing portion 21 passing through the magnetic material portion 51 act as a transformer with the tubing 21 acting as a single turn.

Whilst the winding 52 could have a single turn as well, it is generally found beneficial for this to be a multi-turn winding in the present preferred uses, where very low frequencies are to be used for transmission and reception. This is because the impedance components 53 used to allow generation of impedance in the tubing portion 21 will typically comprise capacitors and the capacitance value of these capacitors necessary to tune the impedance generation means 5 to the required frequency will be smaller where a multi-turn winding 52 is used.

FIG. 3 schematically shows, in circuit diagram form, the impedance generation means 5 and the transformer like arrangement between the tubing portion 21 and the impedance components 53.

FIG. 4 schematically shows in circuit diagram form more detail of the impedance generation means 5 of the present embodiment where the impedance generation means 5 is tuneable. Here the impedance components 53 comprise three capacitors 53 a, 53 b and 53 c which are arranged in parallel with one another and are connected to the winding 52. A first of the capacitors 53 is connected in series with the winding 52 and is so connected at all times. On the other hand the other two capacitors 53 b, 53 c are connectable in parallel across the first capacitor 53 a, but such connection is controlled by respective switches 53 d, 53 e. Thus the impedance generation means 5 is tuneable by the control unit 63 selectively operating the switches 53 d, 53 e to switch the second and third capacitors 53 b, 53 c into and out of the circuit.

Such an arrangement may be used where it is known that a number of predetermined different frequencies will be used for transmission at different times or to allow tuning to obtain the best possible signal in a particular implementation at a particular time. Of course, in reality, a larger number of capacitive elements may be provided as part of the impedance components 53 to give more granular control. Furthermore, rather than using purely passive components, active components can be used to provide similar effects as part of the impedance components 53 in the impedance generation means 5.

It will be appreciated that here what is being done is that the impedance generation means 5 is being used in place of an insulation joint to provide a block or at least significant impedance against signals passing from one portion of the tubing 21 to the other portion of the tubing 21. Of course, in practical terms, using such an impedance generation means 5, it is unlikely to be possible to completely block the signal path between the two portions of tubing 21 on either side of the impedance generation means 51. However, such a complete block is not necessary to give useful results. For example, in the present embodiment a significant enough impedance may be generated in the tubing 21 to allow the sending and receiving of signals.

It will be appreciated that in the bulk metal tubing of the type used in the oil and gas industry then the impedance of a length of tubing will be extremely low. Thus, the impedance which is generated by the impedance generation means may, in absolute terms, be quite low, but still be effective. For example, a signalling circuit in these type of techniques where the metallic flow line is used as a signal channel may have a circuit of impedance of in the order of 5 milliOhms. It has been found that an impedance generation means 5 of the type generally described with respect to FIG. 2 can generate an impedance in the order of 50 milliOhms in the tubing in the region of the impedance generation means. This then is massively higher than the impedance of the same length of tubing 21 without the impedance generation means 5 in place.

It may also be appreciated that even without the windings 52, or impedance components 51, the presence of a piece of magnetic material 51 around the tubing 21 would cause some impedance in the tubing 21. However, this impedance at the low frequencies generally being used to transmit in such systems is generally too small to be useful. The inclusion of suitable impedance components, for example, even a single (correctly chosen) capacitor in series with suitable windings 52 on the magnetic material 51 can dramatically transform the impedance generated at the tuned frequency. Of course, away from the tuned frequency the impedance generated may be of no practical use, but this does not matter in a wide number of circumstances.

FIG. 5 schematically shows, in circuit diagram form, a simple form of impedance generation means 5 where the impedance component 53 is a single capacitor 53 f of a carefully chosen capacitance value to match the frequency of signals which are to be obstructed by the impedance generation means 5.

FIG. 6 schematically shows a drill stem testing system where data concerning well parameters, for example, pressure are to be transmitted from downhole to the surface. Here drill stem tubing 111 is provided in a main bore 101. A downhole mandrel tool 106 is provided for collecting data readings concerning, for example, pressure and transmitting this data towards the surface. The downhole mandrel tool 106 comprises a communications unit 6 of the same general type of that described above with reference to FIGS. 1 to 5. Thus again, an impedance generation means 5 is provided for generating a local electrical impedance in the drill stem tubing 111 and a transceiver 61 is provided for transmitting and receiving signals into and from the drill stem tubing 111 across the local impedance generated by the impedance generation means 5. When transmitting these signals are injected into the drill stem tubing 111 and travel up towards the surface.

However, in this circumstance due to the achievable signal strength, repeater stations 107 are provided at locations along the drill stem tubing 111 to receive the signals in the drill stem tubing 111, amplify these and reapply them for onwards transmission.

Each repeater station 107 comprises a communications unit 6 of a similar type to that described above. Here, however, in order to increase the effectiveness of the reception and signalling capabilities, a length of the drill stem tubing 111 is provided with an outer insulating coating 111 a for a region along either side of the impedance generation means 5 and electrical contact to the drill stem tubing 111 for the application and the reception of signals is made at the remote ends of these insulated portions.

A surface transceiver 108 is provided for receiving the signals transmitted by the downhole mandrel tool 106 after having been passed up the drill stem 111 via the repeating stations 107. The surface transceiver 108 has one terminal connected to the drill stem tubing 111 and another terminal connected to ground.

FIG. 7 shows another situation where a impedance generation means 5 of the type described above with reference to FIGS. 2 to 5 may be used. Here there is a subsea well 201 including a communications system in which signals from downhole are transmitted towards the surface along the metallic structure 211 of the well 201. These signals are detected at the seabed by a seabed transceiver 208 which has one connection to the metallic structure 211 of the well head/tubing near the surface and one connection to earth. In isolation such a communication system has been proven to work well.

However, this is a subsea well 201 where there is a metallic riser 209 leading to a tethered vessel 291 at the surface. This metallic riser 209 is provided to transport the extracted product to the surface of the water at the tethered vessel 291 and, together, the tethered vessel 291 and riser 209 have to accommodate for changes in water level. At least partly because of this, the riser 209 is generally a massive metal component. It may have a wall thickness of three or four inches. This has the result that signals in this riser 209, for example, environmental noise can significantly effect the reception and transmission of signals between the well 201 (and associated communication tools downhole) and the sub-surface receiver 208. Electrically isolating the riser 209 from the well head would help to alleviate this problem. However, this again is a circumstance where the provision of a physical insulation joint is impractical.

Thus, in the present embodiment an impedance generation means 5 of the same general type described above with reference to FIGS. 2 to 5 is provided around the riser 209. This impedance generation means 5 can be tuned to block, or at least significantly attenuate, signals having frequencies which may interfere with the well communication system. The impedance generation means 5 might be tuned to block signals having a frequency where there is most noise or alternatively, may be tuned to block the signals having frequencies which correspond to transmission frequencies used in the well communication system.

The impedance generation means 5 for use in such a situation would most likely but not necessarily be statically tuned rather than “tuneable”.

FIG. 8 shows a further situation where impedance generation means 5 of the type described above with reference to FIGS. 2 to 5 may be used. Here there is a well having a horizontal completion 301 and including a communications system in which signals are transmitted along tubing 311 of the completion.

The present well also includes a power transmission system for transmitting power from one location on the tubing 311 to others. The power may be used for operating the communications system and/or for other purposes.

A plurality of communications units 306 one of which is shown in FIG. 9 and each of which is similar to that described with reference to FIGS. 1 to 5 are provided at selected locations in the well installation. For the sake of brevity no detailed description of the structure of each communications unit 306 is given here—however it is noted that most aspects of the structure and operation of the communications units 306 are the same as that of the communications unit 6 shown in and described with reference to FIG. 2. The following description will rely on the description of the communications unit 6 above where the structure and operation are the same and make use of the same reference numerals to refer to the corresponding parts of the present communications units 306 and concentrate on the differences.

Each of the communications units 306 comprises an impedance generation means 5 of the type described above to facilitate the application of and/or extraction of signals to and/or from the tubing 311. In this embodiment the locations for the communications units 306 are selected to be ones where it is desired to take measurements of pressure and/or temperature. In general terms the communications units 306 may be located wherever it is on the tubing 311 that there is a desire for data communication—this may be, for example, to monitor a parameter or to remotely control an item.

A master communications unit 307 is disposed on the tubing 311 at a location spaced from the communications units 306. The master communications unit 307 is similar to the communications units 306 but also comprises power generation means 308 for generating power which may be used by the communications system.

Significant power can be required for signalling and/or other operations and providing power at downhole locations is always an issue. The present system aids in this by the provision of what might be termed an integral power transmission system.

In the present embodiment the power generation means 308 comprises a turbine which is driven by flow of product (i.e. oil and/or gas) in the tubing 311. Such a device is preferably located in a region of high flow rate. Furthermore, the provision of such a device to extract energy from the flow of product is likely to be intrusive—for example it may be a hindrance to access to the well by wireline. Thus it is desirable to minimise such interference to normal operating of the well by minimising the number of locations at which a power generation means is located.

In the present embodiment there is a single power generation means 308 provided at the master communications unit 307 and power is fed from there along the tubing 311 to the other communications units 306 as will be described below.

This power transmission system is particularly useful in a well with a horizontal completion as this generally means that there is a significant length of tubing passing through the reservoir R where there is often relatively high resistivity, and power may be efficiently transmitted along that length of tubing.

In the present embodiment each of the communications units 306 has its own power source 361—this may include a back up battery and a rechargeable charge storage unit or just comprise a rechargeable charge storage unit—eg a rechargeable battery, or capacitor based device. However each communications unit 306 is arranged to harvest power from the tubing 311 which is transmitted by the master communications unit 307 to charge this device 361 and perform its main functions.

Each of the communications units 306 has a impedance generation means 5 tuned to generate a high impedance at a selected respective frequency—in this embodiment there are three frequencies f₁, f₂, f₃. Furthermore, the master communications unit 307 is arranged to selectively apply power signals to the tubing 311 at these three frequencies f₁, f₂, f₃.

The control unit 63 in each communications unit 306 is arranged to periodically monitor signals on the tubing 311.

In the absence of a signal having the frequency f₁, f₂, or f₃ assigned to that communications unit 306, the control unit 36 maintains a break in the circuit of the winding 52 and impedance means 53 so that the impedance generation means 5 is non-resonant (i.e. just resistive) such that any power signals on the tubing 311 will pass substantially unimpeded.

However, on detection of a signal having the frequency f₁, f₂, or f₃ assigned to that communications unit 306, the control unit 36 makes the circuit between the winding 52 and impedance means 53 so that the impedance generation means 5 is resonant and offers high impedance to the transmitted signal such that power may be extracted across the impedance generation means 5 by the communications unit 6.

The harvested energy may be then used in making measurements and/or signalling.

The control unit 36 may be arranged to send a signal back to the master communications unit 307 when its power requirements have been satisfied.

In an alternative rather than signals being transmitted at different frequencies for each communications unit 306, a different mechanism may be used to control whether a particular communications unit 306 should be harvesting power. For example each communications unit 306 may extract power at a chosen time, in response to a chosen signal (e.g. an address), on detecting that no other unit is extracting power or so on.

The master communications unit 307 may apply signals to and/or extract signals from the tubing 311 across an impedance generation means 5, or in a different way—for example across a conventional insulation joint.

Further it should be noted that a similar, but in most circumstances less preferred, power transmission system might be implemented without the use of any impedance generation means 5. That is to say in an alternative there may be a master communications unit 307 that transmits power along the tubing 311 which is selectively available to a plurality of communications units 306 and one of the above techniques used to decide whether this power can be harvested by a particular communications unit 306. Once such a determination is made, that communications unit 306 may connect in to extract the power. For this purpose each communications unit may be located at a conventional insulation joint which is usually electrically by-passed but can be put in to operation by the respective communications unit 306, when it is desired to signal and/or extract power.

It will be appreciated that the flow line impedance generation means described above are arranged as resonant devices.

As mentioned above the impedance generation means may be arranged to be tuned or tuneable to particular frequencies. Another way of expressing this is to say that the impedance generation means may be arranged to resonate at a predetermined or a selectively variable frequency. In, for example, FIG. 3 the winding 52 and impedance components 53 form a resonant circuit. 

1. Flowline electrical impedance generation apparatus for electromagnetically generating a local electrical impedance in a metallic tubing portion of a flowline, wherein the impedance generation apparatus is one of tuned and tuneable to generate a maximum impedance to signals at a chosen frequency.
 2. Flowline electrical impedance generation apparatus according to claim 1 which comprises a generally toroidal portion of magnetic material for surrounding the tubing portion.
 3. Flowline electrical impedance generation apparatus according to claim 2 in which a winding is provided on the toroidal portion of magnetic material.
 4. Flowline electrical impedance generation apparatus according to claim 3 in which the winding is connected to at least one impedance component which is chosen so that the impedance seen in a tubing portion passing through the toroidal portion of magnetic material varies with frequency.
 5. Flowline electrical impedance generation apparatus according to claim 4 in which the impedance component comprises a capacitor connected in series with the winding.
 6. Flowline electrical impedance generation apparatus according to claim 4, in which the impedance generation apparatus comprises a plurality of impedance components which are selectively electrically connectable with the winding to alter a resonant frequency of the system and hence tune the local electrical impedance.
 7. Flowline electrical impedance generation apparatus according to claim 1 which is arranged to be mounted on the tubing portion such that tubing may run uninterruptedly through or past the impedance generation apparatus when installed and in use.
 8. Flowline electrical impedance generation means according to claim 1 which comprises a control unit for controlling the generation of the local electrical impedance.
 9. Flowline communication apparatus for use where metallic tubing of a flowline system is used in a signal path, the apparatus comprising: impedance generation apparatus for electromagnetically generating a local electrical impedance in a metallic tubing portion of a flowline in the flowline system, wherein the impedance generation apparatus is one of tuned and tuneable to generate a maximum impedance to signals at a chosen frequency; and a communications unit comprising at least one of a transmitter for transmitting signals into the tubing portion across the local electrical impedance and a receiver for receiving signals across the local impedance from the tubing.
 10. Flowline communication apparatus according to claim 9, which comprises a control unit for controlling the impedance generation apparatus to control the local electrical impedance.
 11. Flowline electrical impedance generation means according to claim 8, in which the control unit is arranged to tune the impedance to the frequency of signals being sent and/or received.
 12. Flowline electrical impedance generation means according to claim 8, in which the control unit is arranged to selectively enable and disable the impedance generation apparatus to control whether a local electrical impedance is generated.
 13. Flowline communication apparatus according to claim 11 in which the impedance generation apparatus is arranged to generate an impedance which is tuned or tuneable to the frequency of the signals to be transmitted and/or received across the local impedance.
 14. Flowline communication apparatus according to claim 10, in which the control unit is arranged to measure one of: received signal strength from the tubing and the local electrical impedance in the tubing, and arranged to control the impedance generation apparatus so that at the signalling frequency, the signal strength or impedance respectively tends towards a maximum.
 15. Flowline communication apparatus according to claim 9 comprising a spaced pair of electrical contacts for contacting with tubing so as to connect the transmitter and/or receiver across the local impedance.
 16. A flowline arrangement comprising a metallic tubing portion and impedance generation apparatus as claimed in claim
 1. 17. A flowline communication arrangement comprising flowline communication apparatus as claimed in claim 9 and a length of tubing.
 18. A flowline arrangement according to claim 16, in which the impedance generating apparatus is mounted on or around tubing, and insulation is provided on the outer surface of the tubing in regions on both sides of the impedance generating apparatus or where the tubing on which the impedance generating apparatus is mounted is itself provided within a second length of tubing, insulation is provided between the two lengths of tubing in regions on both sides of the impedance generating apparatus.
 19. A flowline communication arrangement according to claim 17 disposed for communicating between metallic tubing in a main bore hole and metallic tubing in a lateral which is not electrically connected to the metallic tubing in the main bore.
 20. A flowline communication arrangement according to claim 17 disposed for use in drill stem testing.
 21. A flowline arrangement according to claim 16, in which the impedance generation apparatus is provided in a well installation including a communication system arranged to transmit signals at a predetermined frequency and having a riser leading away from a well head, the impedance generation apparatus being disposed around the riser.
 22. A flowline power transmission apparatus set comprising a master unit arranged for applying power to metallic tubing of a flowline system and at least one other unit arranged to extract power from metallic tubing of a flowline system, the at least one other unit comprising an impedance generation apparatus for electromagnetically generating a local electrical impedance in a metallic tubing portion of a flowline, wherein the impedance generation means is tuned or tuneable to generate a maximum impedance to signals at a chosen frequency and is arranged to generate an impedance across which power can be extracted in use.
 23. A flowline power transmission apparatus according to claim 22 in which the master unit is arranged to selectively apply power signals having one of a plurality of predefined frequencies.
 24. A flowline power transmission apparatus according to claim 23 in which there is a plurality of said other units each of which has an assigned predefined frequency and each being arranged to extract power only when a signal having the respective predefined frequency is detected at that unit.
 25. A flowline power transmission apparatus according to claim 23 in which each other unit is arranged to activate the impedance generation apparatus to allow the extraction of power on detection of a signal having a respective predefined frequency.
 26. A flowline power transmission apparatus according to claim 22, in which the master unit comprises a power generation source.
 27. A flowline power transmission system comprising a flowline power transmission apparatus set according to claim 22 and a flowline on which the apparatus set is installed.
 28. Flowline communication apparatus for use where metallic tubing of a flowline system is used in a signal path, the apparatus comprising a toroidal portion of magnetic material for location around a length of metallic tubing and a winding wound around the toroidal portion of magnetic material and connected to at least one impedance component chosen so that an impedance seen in a metallic tubing portion passing through the toroidal portion of magnetic material varies with frequency and a communications unit comprising at least one of a transmitter for transmitting signals into the length of metallic tubing across a portion of the tubing passing through the toroidal portion of magnetic material and a receiver for receiving signals, from the length of metallic tubing, across a portion of the tubing passing through the toroidal portion of magnetic material.
 29. A downhole communication system for communicating between metallic tubing in a main bore hole and metallic tubing in a lateral bore hole which is not electrically connected to the metallic tubing in the main bore, comprising a transmitter for applying signals to the metallic tubing in the main bore so that signals pass into the surrounding formation and towards the lateral bore and flowline communication apparatus according to claim 28 provided in the lateral bore for extracting signals from the tubing in the lateral bore, the signals in the tubing in the lateral bore having been generated by the signals passing through the surrounding formation.
 30. A drill stem testing system comprising, a length of metallic tubing supporting a drill bit, a downhole sensor for sensing a downhole parameter, and a communication system for transmitting data from the downhole sensor to the surface, the communication system comprising flowline communication apparatus according to claim 28 for injecting signals representing said data into the drill supporting metallic tubing for transmission towards the surface.
 31. Flowline electrical impedance generation means for electromagnetically generating a local electrical impedance in a metallic tubing portion of a flowline, wherein the impedance generation means is tuned or tuneable to generate a maximum impedance to signals at a chosen frequency.
 32. A downhole communication system for communicating between metallic tubing in a main bore hole and metallic tubing in a lateral bore hole which is not electrically connected to the metallic tubing in the main bore, comprising transmitting means for applying signals to the metallic tubing in the main bore so that signals pass into the surrounding formation and towards the lateral bore and flowline communication apparatus according to claim 28 provided in the lateral bore for extracting signals from the tubing in the lateral bore, the signals in the tubing in the lateral bore having been generated by the signals passing through the surrounding formation. 